Polypropylene has been since long extruded into sheets and films, depending on the thickness. Of particular interest are those film products that are subjected to orientation techniques and that show improved strength and clearness. Of particular interest are the biaxially oriented polypropylene (BOPP) films that are widely used in the packaging industry and in other applications. BOPP films are particularly suitable for food packaging applications in view of their set of properties: excellent barrier to moisture, resistance to grease and oil, to abrasion, good stiffness, tensile strength, impact and flexibility at low temperatures, gloss, printability and crispness.
Despite polypropylene films and sheets has been known for decades, there is still a wish to improve its properties. Also, due to recent regulatory restrictions on phthalates, it is desirable to make available polypropylene films and sheets that are free from phthalate residues coming from typical Ziegler-Natta catalysts used for their preparation.
International Application WO 2004/013193 discloses, in a comparative example, a bi-oriented film of a polypropylene prepared with a phthalate-free catalyst. While that film shows good mechanical properties, it is not a commercially interesting product due to the low content of stereoblocks of the polypropylene. In fact, film processing performance of the resin is known to be correlated with the amount of the stereoblock component that is crystalline and melts at a significantly lower temperature than the isotactic component.